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Devonian
Morocco
This cross-section of an ammonite has been polished to show the internal chambers and the minerals deposited within them. The gray is mudstone, the white is calcite, and the brown is barite. The latter two are one of the most common pairs of minerals found in fossils. Calcite, the more usual, is invariably deposited first, with barite following it and often replacing it. Note the black and white tubular structure running along the dorsum of the ammonite. This is the siphon, the organelle responsible for maintaining the proper proportion of fluid and gases in the chambers in order to keep the animal upright.